Experimental approaches are proposed to elucidate the biochemistry, innervation and role of pineal organ in regulating brain function, neuroendocrine and behavioral phenomena in fishes. An assessment of both photoreceptive and possible endocrine capacities of the pineal is contemplated with due cognizance of the need for correlating pineal and brain metabolism. The study is considered to provide significant insight into functional evolution of pineal organs in vertebrates. The nature and central associations of pineal innervation will be studied with the method of Fink and Heimer (1967) and the fluorescence histochemical technique of Falck and Owman (1965). Electroencephalography (EEG) and a modification of the photosensor method will be used to examine the role of pineal in the regulation of brain function and locomotor activity respectively. Endogenous 5- Hydroxytryptamine and melatonin metabolism in the pineal and brain will be determined fluorimetrically and the data will be correlated with EEG and locomotor activity patterns to assess a possible role of the pineal in the control of "sleep" and wakefulness in fishes. Protein metabolism will be studied with the method of Lowry et. al. (1951) and on the basis of uptake and incorporation of labelled amino acids. The role of pineal in the control of hypothalamo-hypophyseal-gonadal axis will be investigaged. Effects of pinealectomy and enucleation on the hypothalamus and pituitary will be examined by cytological methods and by Falcks method for monoamines. Modification of gonadal activity will be determined by estimation of gonadosomatic index. Selection of the several species to be used is based on particular regard to pineal morphology and suitability in the study of the various aspects outlined above.